(1) Field of the Invention
This invention relates to optical image information detecting apparatus used for obtaining two-dimensional images of an explosion or combustion (two-dimensional optical images) or of a spectrum in time of fluorescence annihilation (one-dimensional optical images). More particularly, the invention relates to a technique of high-speed photography for use in photographing high-speed phenomena.
(2) Description of the Related Art
Optical image information detecting apparatus for detecting high-speed phenomena include a high-speed image sensor for photographing high-speed phenomena at high speed. Such high-speed phenomena include an explosion, destruction and combustion, for example. As an apparatus for photographing such phenomena, a high-speed photographic apparatus (high-speed video camera) with a high-speed image sensor is used in scientific measurement. The photographing speed of the image sensor usually is about 30 frames per second. However, in high-speed photography for photographing a high-speed phenomenon, a photographing time for one frame is one microsecond, for example, that is a photographing speed at one million frames per second.
FIG. 1 is a schematic view showing an image sensor used in a conventional high-speed photographic apparatus. As shown in FIG. 1, the image sensor has photosensitive units 101, storage units 103, vertical transfer units 105, a horizontal transfer unit 107 and an amplifier 109. Each photosensitive unit 101 is formed of a photoelectric converter, typically a photodiode. Each storage unit 103 has a plurality of cells formed of CCDs (Charge Coupled Devices) connected in series. An electric signal for one pixel is acquired by one photosensitive unit 101 and stored in one storage unit 103 (as enclosed in a dotted line in FIG. 1). Thus, an 80,000-pixel image sensor includes 80,000 pairs of photosensitive units 101 and storage units 103 in a two-dimensional arrangement.
The photosensitive unit 101 converts incident light into an electric signal in each electronic shuttering cycle, i.e. at an electronic exposure time. After the exposure time, the electric signal converted is delivered to a first cell Ch of the storage unit 103 through a signal fetch gate not shown. At this time, the electric signal is moved by forming a potential gradient over the photosensitive unit 101, signal fetch gate and first cell Ch. After the signal delivery, an exposure time starts and the photosensitive unit 101 generates an electric signal again. The storage unit 103 forwards electric signals stored in the cells to next adjacent cells synchronously with the above signal delivery. Upon completion of a series of photographic operations, the vertical transfer units 105 and horizontal transfer unit 107 fetch the electric signals stored in the storage units 103.
In this way, the image sensor for high-speed photography having the storage units 103 can fetch electric signals en bloc after a photographing operation, which would otherwise take a relatively long time. It is therefore possible to increase the photographing speed to a rate of transfer of the electric signals from the storage units 103.
In the image sensor for high-speed photography, each photosensitive unit 101 has a large area compared with each cell of the storage unit 103. This construction is adopted to provide a high open area ratio (fill factor) in order to secure a sufficient quantity of light incident on the photosensitive unit 101 in a short electronic shuttering cycle (exposure time). In order to arrange, without a gap, the photosensitive unit and storage unit which differ in size and shape, an actual image sensor is devised to set the storage unit aslant. Such an image sensor is disclosed in Japanese Unexamined Patent Publication No. 2001-169189, for example.
However, signal transfer cannot be performed smoothly in the case of a high-speed photography at such a high rate as one million frames per second. Specifically, the following problems (I) and (II) are encountered:
Problem (I)
Photographing speed is variable with the electronic shuttering cycle, i.e. exposure time. When the electronic shuttering cycle is shortened, the photographing speed will become fast. However, when the electronic shuttering cycle is shortened, the transfer rate described above must also be raised. Unless the transfer rate is raised, the electric signals will stagnate instead of being transferred, and the photographing speed cannot exceed the transfer rate. However, if the transfer rate is raised by increasing clocking speed, the storage units 103 will generate heat which could cause inconveniences and in some cases to the extent of destroying the devices. Thus, the transfer rate cannot be increased easily. The photographing speed is determined by the transfer rate, and cannot be increased.
Problem (II)
Electric signals cannot be fetched (delivered) completely from the photosensitive units 101 to the first cells Ch of the storage units 103. As a result, the electric signals generated at a preceding exposure time remain in the photosensitive units 101 until a next exposure time, thereby causing an afterimage. This is considered due to the area of each photosensitive unit being large compared with that of each cell, which makes it impossible to form a proper potential gradient from the photosensitive unit to the cell. Since each photosensitive unit has a large area, electric charges moving even at a constant speed would only consume time. As a result, the signals cannot be transferred smoothly, thereby causing an afterimage.